1. Field of the Invention
The present invention relates to a light-scanning optical system, having a wobble-correcting function, for scanning a laser beam converged on a surface of a photosensitive material, so as to form an image; and a light-scanning apparatus using the same. In particular, the present invention relates to a light-scanning optical system having a wobble-correcting function applicable to a system with a long scanning length exceeding the shorter side of an A3-size (Japanese Industrial Standard) sheet, and a light-scanning apparatus using the same.
2. Description of the Prior Art
There have conventionally been known various kinds of laser-scanning optical systems for scanning a laser beam converged on a surface of a photosensitive material, so as to form an image, and laser printer apparatus and laser printmaking apparatus using these optical systems.
Among them, known as a light-scanning optical system having a so-called wobble-correcting function are those disclosed in commonly-assigned Japanese Unexamined Patent Publication Nos. 4-141619 and 5-215986. Here, the light-scanning optical system having a wobble-correcting function comprises a first imaging optical system for linearly forming an image of a luminous flux from a light source onto a light-deflecting/reflecting surface of light-deflecting means or in the vicinity thereof, and a second imaging optical system for forming an image of the luminous flux deflected by the light-deflecting means onto a surface to be scanned, wherein the light-deflecting/reflecting surface and the surface to be scanned are substantially optically conjugate with each other with respect to a cross section in a sub-scanning direction.
In each of the light-scanning optical systems disclosed in the above-mentioned publications, the second imaging optical system is composed of two lens elements, whereby the amount of curvature of field is made smaller, and compactness is realized in the apparatus.
However, these light-scanning optical systems having a wobble-correcting function and light-scanning apparatus using the same are only applicable to systems with a relatively short scanning length, such as one corresponding to the shorter side of an A4-size Japanese Industrial Standard) sheet (210 mm).
Also, even when a scanning length on such an order is sufficient, they cannot be employed in a system requiring a higher resolution.
In the above-mentioned light-scanning optical systems and light-scanning apparatus using the same, the absolute value of imaging magnification between the light-deflecting/reflecting surface and the surface to be scanned in the cross section in the sub-scanning direction is as high as about 5. Therefore, when they are employed in a system having a longer scanning length or higher resolution, the curvature of field in the sub-scanning direction (sagittal) component becomes so large that it may be difficult to form uniform spots on the surface to be scanned. Also, since the asymmetry in curvature of field generated due to light obliquely incident on the light-deflecting/reflecting surface is further enhanced in the surface to be scanned, the uniformity of light spots would deteriorate.
Further, since the deviation in the vicinity of the light-deflecting/reflecting surface is enhanced, the positional precision of the light-deflecting means must be kept at a high degree.
The above-mentioned magnification can be lowered if the second imaging optical system can be positioned farther from the light-deflecting/reflecting surface between the light-deflecting/reflecting surface and the surface to be scanned. In this case, however, if a scanning length greater than the shorter side of an A4-size Japanese Industrial Standard) sheet is contemplated, then each lens of the second imaging optical system becomes so large that the making of lenses with a required precision may be quite difficult, thus increasing the cost.